1. Field of the Invention
The present invention relates to magnetic powder for magnetic recording.
The present invention further relates to a magnetic recording medium comprising the above magnetic powder for magnetic recording as ferromagnetic powder in a magnetic layer, and to a method of manufacturing the above magnetic powder for magnetic recording.
2. Discussion of the Background
Hexagonal ferrite is widely employed as magnetic powder for magnetic recording. The coercive force thereof is great enough for use in permanent magnetic materials. The magnetic anisotropy that is the basis of the coercive force derives from its crystal structure. Thus, high coercive force can be maintained even when the size of the particles is reduced. Further, magnetic recording media employing hexagonal ferrite magnetic powder in a magnetic layer have high density characteristics due to the vertical component. Thus, hexagonal ferrite is ferromagnetic powder that is suited to high density recording.
In recent years, recording densities have been increasing in the field of magnetic recording. To achieve an accompanying reduction in noise, there has been a need to reduce the particle size of hexagonal ferrite.
However, when the size of hexagonal ferrite magnetic particles is reduced, the energy maintaining the magnetic particles in the direction of magnetization (magnetic energy) tends to be difficult to resist thermal energy. So-called thermal fluctuation ends up causing recording retention property to drop, and the phenomenon whereby magnetic energy is overcome by thermal energy and recording is lost can no longer be ignored. This point will be described in greater detail. “KuV/kT” is a known index relating to the thermal stability of magnetization. Ku is the anisotropy constant of a magnetic material, V is the particle volume (activation volume), k is the Boltzmann constant, and T is absolute temperature. When the magnetic energy KuV is increased relative to the thermal energy kT, it is possible to inhibit the effect of thermal fluctuation. However, the particle diameter V, that is, the particle size of the magnetic material, should be kept low to reduce the noise of the medium, as set forth above. Since the magnetic energy is the product of Ku and V, as stated above, it suffices to increase Ku to increase the magnetic energy when V is in the low range. However, the relation HK=2Ku/Ms exists between Ku and the anisotropy field HK. When Ku is increased without a change in Ms, HK also increases. The anisotropy field HK is a magnetic field intensity that is necessary to achieve saturation magnetization from the direction of the hard axis of magnetization. When HK is high, the reversal of magnetization by the magnetic head tends not to occur, recording (the writing of information) becomes difficult, and the reproduction output ends up dropping. That is, the higher the Ku of the magnetic particle, the more difficult it is to write information.
As set forth above, it is extremely difficult to satisfy all three characteristics of higher density recording, thermal stability, and ease of writing. This is known as the trilemma of magnetic recording. It will be a major problem in achieving higher density recording in the future.
On the other hand, barium ferrite is widely employed as hexagonal ferrite for use in magnetic recording. However, strontium ferrite is known to have a higher Ku and a higher σs than barium ferrite. Since HK=2Ku/Ms and Ms=σs×ρ (ρ: specific gravity), strontium ferrite is a magnetic material that is advantageous for resolving the trilemma by achieving a low HK with a high Ku. In this regard, Japanese Unexamined Patent Publication (KOKAI) No. 2013-211316 or English language family member US2013/256584A1, which are expressly incorporated herein by reference in their entirety, proposes a method of manufacturing strontium ferrite that is suitable as magnetic powder for high-density recording.